Design Practices for Rock Slopes and Rockfall Management (2022)

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13   State of the Practice The state of the practice described in this synthesis is determined with a questionnaire designed for completion in about 30 minutes. The questionnaire was administered with an Internet-based survey tool and disseminated to lead geotechnical personnel in all 50 state DOTs, Puerto Rico, Washington D.C., and the offices of Western, Central, and Eastern FLHD (WFLHD, CFLHD, and EFLHD). The questionnaire was subdivided into five categories focusing on (a) rock slope and rockfall statistics, (b) design standards, (c) design practice, (d) performance measures, and (e) lessons learned. Each category had several questions, for a total of 30 questions specific to rockfall and rock slope design. The state of the practice for each category is described in the following sections. Appendix A contains the questionnaire, and Appendix B contains compiled response data. Individual responses are contained in Appendix C. Respondents were given an opportunity to upload policy documents or drawings, which are contained in Appendix D. Questionnaire Responses There were 52 survey respondents out of 55 requests (a 95% response rate). Responses were requested from 52 DOTs, and 49 responded (94% response rate). In addition, 100% of the three FLHD offices provided responses. Two questions were given that permitted DOTs to respond to the survey without excessive time, as not all states possess the topography and geology that requires frequent rockfall and rock slope design. Of the total 52 responses to the survey, 34 (65%) had sufficient experience with rockfall and rock slope design to continue with the survey. Figure 7 shows which DOTs responded to the survey and which ones with sufficient rock slope experience continued the survey. Respondents were primarily geologists or geotechnical engineers, either in a supervisory or managerial role. DOTs that responded with sufficient experience are described herein as “rock slope” state DOTs and agencies, and response statistics relevant to design efforts are relative to these 34 rock slope departments. Rock Slope and Rockfall Occurrence and Performance State DOTs have varying counts of rock slopes to maintain. Inventory systems also differ with regard to both inventory completeness and entry criteria. Of the 34 rock slope departments, most maintain under 100,000 miles of roadway, with an average of 33,400 miles. Along the maintained length of highways, about 50% of DOTs have under 500 rock slopes to maintain along that length, with about 30% having 1,000 to 5,000. Note that the inventory quantities may be based on judgment and experience rather than comprehensive inventory programs. From a review of questionnaire results, a ratio, or relative occurrence of rock slopes per 100 miles, can be derived from more mountainous states with mature rock slope inventory C H A P T E R 3

14 Design Practices for Rock Slopes and Rockfall Management programs. These states’ rock slope density ranges from approximately 18 to 50 rock slopes per 100 miles of highway. Those states with relatively less topographic relief but mature inventory programs, such as Tennessee, Ohio, and New York, return densities of 6 to 13 rock slopes per 100 miles of highway. About 37% of respondents indicated that the number was estimated based on a comprehensive inventory of rock slopes across their entire highway system, with about 30% of respondents saying this was estimated based on personal experience. Survey respondents were queried regarding current rock slope and installed rockfall mitiga- tion system performance along the DOT’s roadways. The survey resulted in wide performance distributions for slopes that have not required maintenance and those that require only periodic maintenance. Degradation of rockfall mitigation measure performance was found to be infre- quent among most DOTs, but a few outliers reported up to 20% of their rockfall mitigation systems are no longer meeting the original intent. Figure 8 contains box and whisker plots for statistical information. Note that this figure contains explanatory text for interpreting many plots contained in this report. Design Efforts and Standards DOTs responded to how often designers of various affiliations were used by their agency. Most designs were provided by an agency’s in-house professionals or consultants for both rock- fall mitigation and rock slope design (Figure 9). Rockfall mitigation required more frequent design efforts by in-house personnel due to the emergency response typically required following rockfall events. Specific federal or AASHTO design requirements for rockfall and rock slope design do not exist beyond the FS guidance previously described. This synthesis project aimed to document Figure 7. Map of survey respondents and DOTs who had enough experience with rockfall and rock slope design to continue the survey.

State of the Practice 15   current DOT design standards, when they may have come into eect, their formality, and their future policy development plans. Of the rock slope respondents, all had some sort of informal or formal design standards in place or are considering adopting formal standards with docu- mentation. Of the 33 DOTs, 14 (42%) responded that they had formal standards for both rockfall and/or new rock cut slopes that have been documented as policy (Figure 10). Another 14 (42%) have undocumented standards that may have been communicated among DOT personnel and/or applied on a project-specic basis. e remaining ve (15%) use consistent design approaches. Nearly half (47%) of DOTs responded that they have unique design standards for new rock cuts and for rockfall mitigation, whereas one-fourth indicated they were the same (Figure 11). Another 16% responded that they have design standards for new rock cuts only, and no respond- ing DOTs have design standards for rockfall mitigation only. 1. Rock slopes that have not required maintenance or produced rockfall hazards since original construction 2. Rock slopes that have produced rockfall and required periodic maintenance 3. Rock slopes that have rockfall mitigation measures installed and are functioning as originally designed 4. Rock slopes that have rockfall mitigation measures installed but measures are not functioning as originally designed 5. Rock slopes that have been reconstructed or minorly realigned in the past 25 years using modern design approaches and guidance 6. Rock slopes that have been newly constructed for a new roadway or major realignment in the past 25 years using modern design approaches and guidance Number of responses (n) = 32 (a) (b) 1. Design services provided by agency in-house professionals 2. Design services provided by consultants 3. Design services provided by a construction contractor 4. Design services provided by a manufacturer n = 33 Figure 8. Distribution of mitigation and maintenance of currently installed rock slopes and rockfall mitigation systems along the DOTs’ roadways. Plot contains explanatory annotations. Figure 9. Frequency of design providers used by DOTs for (a) rock cut slopes and (b) rockfall mitigation measures.

16 Design Practices for Rock Slopes and Rockfall Management The survey asked participants if their DOT standards differed based on road classification (e.g., highway system, functional classification) or other metrics (e.g., traffic volume), as is common among design standards for other aspects of highway design, such as curve radii or clear zone requirements. Of the respondents, 42% indicated that their DOT standards are consistent across roadway classification, whereas another 42% indicated that standards differ based on road classification or other metrics. The most common “other metric” was indicated to be site specific or partner/contractor specific. See Figure 12 for additional breakdown. Question commentary indicated that in practice the secondary road systems receive more customization due to generally lower speed limits and lower traffic volumes, whereas other practical factors, such as magnitude of the rockfall hazard and/or funding availability, factor into applicability of design goals. Design Standard: Ditch Containment Percentages Of the 21 DOTs that have adopted standards for rockfall mitigation and rock cut slope design, 16 provided specific design standards. In general, these responses apply to highways with high performance expectations, such as interstates, primary arterials, or the National Highway System (NHS). All respondents who gave one classification indicated that the percentage of rockfall contained by roadside ditch, percentage of rockfall impacting the roadway ditch, and reduc- tion percentage of rockfall reaching the road must be 90% or greater. One DOT indicated a containment percentage of 100% (VA); one cited 98% containment (UT); one, 95% to 98% (MT); 1. Yes, formal standards have been developed and documented. 2. Yes, standards have been developed but not documented as formal department policy. 3. Yes, informal standards have been communicated between agency geotechnical personnel. 4. No, but we are considering formal, documented design standards. 5. Yes, informal standards that may be applied on a project-specific basis. 6. No, we have no design standards, but we use consistent approaches to develop rock slope and rockfall mitigation designs. n = 33 1. Yes, design standards are unique between designs for new cuts and rockfall mitigation. 2. No, rock slope design standards are the same for rockfall mitigation as they are for new cuts. 3. No, we only have design standards for new cut slopes. 4. Not applicable; we do not have rock slope or rockfall design standards. 5. No, we only have design standards for rockfall mitigation. n = 33 Figure 10. Survey responses for agencies’ development of design standards guiding rock slope and/or rockfall mitigation. Figure 11. Survey responses regarding whether an agency’s design standards differ between new cuts and rockfall mitigation of existing slopes.

State of the Practice 17   four, 95% (OH, NJ, WY, PA); and seven, 90% (ID, ME, NC, NY, TN, WI, WFLHD). One agency (CFLHD) uses a scoring threshold that equates to Good Ditch Effectiveness in a scoring system. One agency’s response was not applicable. DOTs were given the opportunity to indicate how design standards may change between classifications. Classifications were supplied by the responding DOT. When provided, the standards changed only slightly between the two. One DOT indicated the largest change in standards with the first classification given as “new rock cut” and second classification “existing rock cut.” The percentage of containment by the roadside ditch dropped from 90% for new cuts to 80% for existing cuts, and the percentage of rockfall impacting the roadway ditch likewise dropped from 97% to 90%. Appendices B and C contain a full breakdown of responses. Design Standard: Ditch Width In some instances, a minimum width of the roadside ditch is specified. This is effectively a correlation to ditch effectiveness, as wider ditches contain more rock, although percentages change based on launch features and slope height. Four DOTs provided standard widths that range from 10 to 21 feet. Other responses refer to other ditch functions, such as clear zone or drainage, as primary width considerations. For DOTs that specified two classification standards, minimum ditch width dropped from 18 feet for interstates to 12 to 14 feet for U.S. or state routes or 10 to 6 feet for new versus existing rock cuts. In the absence of ditch width requirements, the geotechnical criteria to determine a suitable roadside ditch width were based on cut height, angle, and discontinuity orientation and the use of the RCAD (Pierson et al. 2001). Six DOTs indicated that ditch width is not standardized and is site specific. Design Standard: Others Some DOTs have or are developing a GAM program or an RHRS that generates rock slope scores. Two DOTs cite specific scores that assist with informing the design approach. Both indicate scores that trigger mitigation efforts or otherwise serve as target scores for mitigation efforts: RHRS scores above 300 or those above 650 in a state-specific rating rubric. Frequency of rockfall debris reaching the roadway and/or frequency of maintenance activity can be useful criteria to guide design. Two DOTs provided measurable criteria for frequency of rockfall reaching the road: once every 5 years or twice per year, respectively. One other DOT stated that it is documented but has no allowable value. Generalized goals for maintenance activity were also cited, ranging from “infrequent” to yearly, to three times per year. 1. No, rock slope design standards are consistent across roadway classifications. 2. Yes, separated by another metric. 3. Yes, separated by highway functional classification (interstate, major arterial, minor collector, etc.). 4. Not applicable; we do not have rock slope or rockfall design standards. 5. Yes, separated by average daily traffic (ADT) volume. 6. Yes, between highway system designation (National Highway System, state highway system, etc.). n = 33 Figure 12. Survey responses for agencies’ variation in design standards based on roadway classification.

18 Design Practices for Rock Slopes and Rockfall Management Probabilistic approaches were cited by two DOTs. Less than 1% probability was noted by one DOT but was not affiliated with a specific consequence, such as roadway impedance, accident, or injury. Another DOT indicated that rockfall probability would be acceptable if debris was limited to the shoulder but not the travel lane. Respondents were asked if their DOT used these same goals for other appurtenant facilities, such as recreational paths, maintenance sheds, and bridge abutments. The response was split, with 34% of rock slope respondents using the same goals for other designs, 29% not using the same goals, and 30% concluding that this was not applicable to their DOT. Before Wyllie’s 1987 Rock Slope Inventory System, development of New York DOT’s RHRS, and the subsequent FHWA programs in the 1990s, if rock slope or rockfall was considered in design, it largely focused on using the Ritchie ditch criteria. This is indicative in the survey results, as 95% of the standards were adopted after 1987 when rock slopes began to be inven- toried with rating systems. Only two states indicate adoption of standards prior to 1987. The largest proportion of design standards were established by DOTs since 2000 (Figure 13). Design Considerations Determining the level of importance between various technical and risk considerations is an important element to consider in the design process. Survey respondents were queried regarding the relative importance of different technical considerations for their DOT when designing rockfall mitigation and new cut slopes. Note that respondents could give factors equal importance. Overall, the technical rankings were similar for rockfall mitigation and new rock cuts. The most important technical aspect for both was constructability, with about 74% (23 of 31 rock slope DOTs responding to the question) indi- cating this as a “most important” component (Figure 14). Long-term maintenance was ranked by most DOTs as a first or second most important technical design aspect for both new rock cuts (84%, or 26 of 31) and rockfall mitigation (81%, or 26 of 32). The overall least important technical consideration ranked by DOTs was project delivery method, with about 25% (8 of 32) indicating rockfall mitigation and about 31% (10 of 32) new cut slopes. Survey respondents indicated the relative importance of various risk considerations for their DOT. Risk consideration rankings were similar for rockfall mitigation and new rock cuts, such as technical considerations previously discussed. The overall most important risk consideration for both new rock cuts and rockfall mitigation was the consequences associated with major slope 1. 1980s 2. 1990s 3. 2000s 4. 2010s n = 22 Figure 13. Survey responses grouped by decade for when DOTs adopted rockfall standards.

State of the Practice 19   failure, with about 94% (31 of 33) of respondents indicating this as a “most important” compo- nent for new rock cuts and about 79% (26 of 33) for rockfall (Figure 15). Consequence of minor slope failure, maintenance frequency, and cost and fiscal impacts of poor performance were also ranked as important aspects. Risk acknowledgment was found to be evenly distributed among rankings, suggesting that DOTs acknowledge this risk factor in various ways. Aesthetic concerns are becoming more important in design (Andrew et al. 2011). The survey addresses this issue by querying the degree to which rock slope DOTs consider aesthetics in the design process for rockfall mitigation and new rock cuts. The most common answer (36%, or 12 of 33 respondents) was considering aesthetics less than 25% of the time (Figure 16). Five (15%) DOTs indicated that they do not consider aesthetics in design at all. Commentary accompanying the responses often referred to working with other agency stakeholders, such as state park departments or the Forest Service, in combination with public input, as the main drivers of aesthetic considerations. Of the DOTs who did consider aesthetics, staining of steel products was often selected, with contouring of wire mesh and removing proximal ends of rock bolts following close behind (Figure 17). Additional aesthetic treatments included staining shotcrete, biomechanical stabilization, or application of color treatments on freshly exposed rock or other elements. Figure 14. Survey responses for agency’s relative importance rankings of technical considerations for rock slope design and rockfall mitigation. N = 27 to 32 (blank responses not counted). Figure 15. Survey responses for agency’s relative importance rankings of risk considerations for rockfall mitigation and new rock cuts. N = 30 to 33 (blank responses not counted).

20 Design Practices for Rock Slopes and Rockfall Management Figure 16. Survey responses for agency’s percentage of projects that consider aesthetics in the design process. Figure 17. Survey responses for agency’s methods used to address aesthetic concerns. 1. 90%–100% 2. 75%–90% 3. 50%–75% 4. 25%–50% 5. Less than 25% 6. Not applicable n = 33 1. Staining of steel products 2. Other 3. Contouring of wire mesh/nets to the slope 4. Removing proximal ends of rock bolts 5. Hammering out half casts on cut slopes n = 28 Design Tools Relative frequency of use for various technical design tools and techniques was evaluated for three dierent design needs: (a) new cut slopes when rock is not exposed, (b) new cut slopes when rock is exposed, and (c) rockfall mitigation on existing slopes. Design tools listed are based on the literature review sources. Figure 18 contains histograms of the relative frequency of use for various technical design tools for each category, ordered by frequency of use. By far, the most used technical design tool by all three categories was surface reconnaissance, which was used 90% to 100% of the time. Design guides and corresponding containment per- centage calculations are widely used by all three categories in design, on average, about 75% to 85% of the time. Geotechnical drilling was used most frequently (on average, 80% of the time) for new cut slopes when rock is minimally or not exposed, with a much lesser frequency for where rock is exposed. Rockfall modeling (2-D), rock mass characterization, and stereonet analyses are used regularly for all three design purposes. Additional tool utilization begins to become more purposeful, using more targeted analytical tools that t the nature of the exposure. For instance, application of point cloud analysis and rope-based reconnaissance eorts increased for rockfall mitigation and rock slopes with exposed

State of the Practice 21   rock outcrops. Likewise, use of geophysical methods, specialized laboratory testing, and instru- mentation is higher where rock is not exposed in outcrop. Performance Measures Rock slope DOTs were asked if they had any performance measures in place specific to rockfall and rock slope management. More than 50% (17 of 32) of these DOTs reported that they do not have performance measures in place, indicating that nearly half (15 of 32) track the performance of rock slopes (Figure 19). The most common tracking was found to be done informally by the internal DOT geotechnical group. Similar results were indicated when query- ing if regular data collection efforts are carried out to determine if performance objectives are met, with 72% of DOTs responding that these efforts are not performed (Figure 20). Lessons Learned To conclude the survey, rock slope DOT respondents shared lessons learned throughout the last 10 to 20 years. Out of these 34 DOTs, 20 responded to this question. DOTs provided insightful responses and coalesced on some common themes throughout. Funding Several DOTs noted the difficulty in identifying sources and obtaining funding for rockfall mitigation and new cut slopes. The apparent high cost to meet more modern design standards can be a deterrent for funding resources. For example, a ditch for an 80-foot-tall 0.5H:1V slope would have to be 120% wider to retain 95% of rockfall versus 50%, resulting in larger excavation 99%97%90% 85 82 81 55 50 46 46 42 31 21 16 13 5 2 83 58 69 40 60 26 44 57 27 13 10 28 15 8 75 27 55 30 68 18 42 53 29 6 7 38 21 5 Note: Full response text is in Appendix B. Figure 18. Average design tool use frequency for new rock cut design and rockfall mitigation. N = 31 to 33.

22 Design Practices for Rock Slopes and Rockfall Management volumes. However, several DOTs observed that over the years the importance and success of rockfall mitigation projects has been recognized by department management and consideration of rockfall for highway projects has increased. Communication The benefits of clear and consistent communication were noted by multiple DOTs. They indicated the importance of good communication with the contractor during the construction phase and design phase so that the final product is up to the DOT’s standards and/or expecta- tions. Several DOT respondents also indicated the importance of routine inspection and mainte- nance being conducted by experienced personnel to ensure that the mitigation infrastructure or a new rock cut slope is performing as intended. Education and open communication between state DOTs familiar with rock slopes and rockfall and less experienced agency partners helps guide informed decision making. For instance, experienced DOTs can establish a peer exchange with those that are less experienced. Approaches, experiences, and key considerations can be quickly passed from those that are more experienced. Some agencies bear the responsibility of providing technical expertise to other agencies (e.g., FHWA serving the National Park Service as a client), and providing technical education to the client agency is an important aspect to developing a response that addresses both hazard and other client goals. Figure 19. Survey responses for whether state DOTs have performance measures specific to rock slopes and/or rockfall management. Figure 20. Survey responses for whether state DOTs regularly engage in data collection efforts to determine if design standards and/or performance objectives are met. 32 1. No. 2. Yes, informally tracked by our geotechnical group. 3. Yes, as part of a larger management program. n = 1. No 2. Yes 3. Not applicable n = 32

State of the Practice 23   Comprehensive Data Acquisition Another common lesson learned was the importance of having a rock slope inventory and comprehensive site characterization of rockfalls and cut slopes for selection and design purposes. GAM and/or RHRS programs can help with funding and directing it to areas where safety or reliability improvements provide the greatest benefit. DOTs indicated that tracking and identifying potential hazards and documenting performance leads to overall safer roadways and facilitates project funding. Recent technological advances are proving to be very helpful for comprehensive site character- ization, particularly for those areas with well-exposed rock outcrops. Unmanned aerial vehicles (UAVs) equipped with cameras and/or survey equipment permit the reconnaissance and measurement of slope features remotely. Repeat data collection efforts permit rapid quantifi- cation of rockfall activity and objective measures of slope performance. Experience Rock slope DOTs indicated that the experience of the designer, contractor, and inspector is a key component of a successful rock slope project. Experienced designers can identify likely constructability issues and account for them during design. Rockfall contractors with experience dealing with the steep, rockfall-prone slopes and the corresponding safety and production issues result in both accurate bids and a safer work zone. Rock slope and rockfall sites change consid- erably during construction activities, and an experienced inspector can adapt the design intent for presented conditions. Technical considerations for new rock cuts, such as blasting methods and reinforcement requirements, can be adjusted as the excavation progresses.